Tax ID Numbers Explained: EIN, SSN, and ITIN for Small Business Owners

Picture this: you are ready to open a business bank account, sign your first client, or hire your first employee — and the form in front of you asks for a "Tax ID Number." You pause. Is that your Social Security Number? Something you apply for? Something your accountant already set up? If you have ever frozen at that question, you are not alone.

A tax identification number is the IRS's way of telling you apart from the roughly 33 million other small businesses in the United States. Using the wrong one — or skipping the step entirely — can delay payroll, trigger penalties, expose your SSN on dozens of invoices, or lock you out of business credit. The good news: getting this right is mostly paperwork, and much of it is free.

This guide walks through every tax ID number you are likely to encounter, who needs which, and how to get them without overpaying a third-party service.

What Is a Tax ID Number?

A Tax Identification Number (TIN) is any unique number the IRS uses to track a person or entity for tax purposes. "TIN" is an umbrella term, not a single number. Under that umbrella sit five specific identifiers:

• Social Security Number (SSN) — for individuals eligible to work in the U.S.
• Employer Identification Number (EIN) — for businesses
• Individual Taxpayer Identification Number (ITIN) — for individuals ineligible for an SSN
• Adoption Taxpayer Identification Number (ATIN) — temporary, for pending adoptions
• Preparer Tax Identification Number (PTIN) — for paid tax preparers

Most small business owners only need to understand the first three. Let's break each one down.

Social Security Number (SSN)

Your SSN is the nine-digit number you probably memorized in childhood. It is issued by the Social Security Administration (SSA), not the IRS, but the IRS still uses it as your primary tax identifier.

Who gets one

SSNs are available to U.S. citizens, lawful permanent residents (green card holders), and non-citizens with authorization to work in the United States. You apply at an SSA office using Form SS-5, with a birth certificate, passport, or similar proof of identity.

When a business can use an SSN

If you are a sole proprietor with no employees, no separate business entity, and no retirement plan obligations, you can technically use your SSN on your Schedule C tax return. The same applies to single-member LLCs that have not elected corporate taxation and have no employees — the IRS treats them as "disregarded entities," so tax filings flow through your personal return.

Why you should rarely stop there

Relying on your SSN for business feels convenient until you realize how many people end up holding it. Every client who pays you more than $600 will request a W-9 with your tax ID on it. Every payment processor, every vendor, every bank will file it somewhere. That's a lot of places for your personal number to surface.

Most single-member LLC owners eventually get an EIN anyway, either because a bank requires one to open an account or because they don't want their SSN traveling around the economy.

Employer Identification Number (EIN)

An EIN is a nine-digit number formatted as XX-XXXXXXX, assigned by the IRS to a business entity. Think of it as a social security number for your company.

Who is required to have one

The IRS requires you to obtain an EIN if any of the following apply:

• You have one or more employees
• You operate as a corporation (C-corp or S-corp) or partnership
• You file employment, excise, or alcohol/tobacco/firearms tax returns
• You withhold taxes on income paid to a non-resident alien
• You have a Keogh plan or other qualified retirement plan
• You are involved with certain trusts, estates, non-profits, or farmers' cooperatives

Multi-member LLCs always need an EIN because the IRS treats them as partnerships by default.

Who should get one even if not required

Sole proprietors and single-member LLCs without employees can skip the EIN legally — but most still get one because it is:

• Required by most banks to open a business checking account
• Requested by clients filling out 1099s (many refuse to work with a bare SSN)
• Essential for business credit — lenders build your Dun & Bradstreet profile around your EIN
• A privacy shield — your SSN stays off W-9s, invoices, and vendor paperwork
• A future-proof step — the day you hire your first contractor or employee, you already have the number you need

How to apply (free, 15 minutes)

Ignore the third-party sites charging $79 or $149 to "file" your EIN. Applying directly with the IRS is free, fast, and simple:

1. Visit IRS.gov/EIN and click "Apply Online Now"
2. Confirm your principal business is in the U.S. or U.S. territories
3. Answer questions about your entity type, ownership, and reason for applying
4. Submit the application and print the confirmation letter (CP 575)

A few practical notes:

• The online session cannot be saved and expires after 15 minutes of inactivity. Gather your information before starting.
• The "responsible party" must be a real person with an SSN or ITIN — not another entity.
• You can only submit one EIN per responsible party per day.
• The system is only available Monday through Friday, 7 a.m. to 10 p.m. Eastern.
• If you don't have an SSN or ITIN (common for international founders), you must apply by fax or mail using Form SS-4. Expect four to six weeks.

Save the EIN confirmation letter somewhere you can find it. You will need it to open bank accounts, apply for state licenses, and prove your entity's existence for years to come.

Individual Taxpayer Identification Number (ITIN)

An ITIN is a nine-digit number the IRS issues to individuals who have a U.S. tax filing obligation but cannot get an SSN. Common examples include non-resident aliens with U.S. rental income, foreign spouses of U.S. citizens, and dependents of visa holders.

What an ITIN does — and doesn't — do

An ITIN does:

• Allow you to file a U.S. tax return
• Claim certain tax treaty benefits
• Appear on W-9s for foreign contractors working with U.S. clients

An ITIN does not:

• Authorize you to work in the United States
• Make you eligible for Social Security benefits
• Qualify you for the Earned Income Tax Credit

How to apply

File Form W-7 along with your federal tax return (unless you qualify for an exception) and original or certified-copy identity documents. A valid passport covers both identity and foreign status in a single document. Without a passport, you will need at least two documents from the IRS-approved list.

Three submission methods:

• Mail everything to IRS ITIN Operations in Austin, Texas
• Work with a Certified Acceptance Agent (CAA), who can verify your documents and spare you from mailing originals
• Visit an IRS Taxpayer Assistance Center by appointment

Processing typically takes 7 to 11 weeks. Submit at least 45 days before you need to file.

ITIN renewals

ITINs expire after three consecutive tax years of non-use. If yours has expired — or if it was issued before 2013 with certain middle digits (88, 90, 91, 92, 94, 95, 96, 97, 98, 99) — you must renew using the same W-7 process. Filing with an expired ITIN delays refunds and can disallow credits.

From ITIN to EIN

If you are a non-resident founder, you don't need an SSN to get an EIN for your U.S. business. You can apply by fax (typically four business days) or mail (four to six weeks) using Form SS-4, listing your ITIN or leaving "Foreign" in the SSN field. This is how most international entrepreneurs launch U.S. LLCs and C-corps.

ATIN and PTIN: Brief Mentions

Adoption Taxpayer Identification Number (ATIN) is a temporary nine-digit number for adoptive parents who cannot get the child's SSN in time to file their tax return. Apply with Form W-7A. It's valid for two years and is replaced by the child's SSN once issued.

Preparer Tax Identification Number (PTIN) is required for any paid tax preparer who signs a federal return. If you are not in the tax preparation business, you will never need one — but knowing it exists helps when you vet your own accountant. A legitimate preparer's PTIN appears on every return they sign.

State Tax ID Numbers

Federal tax IDs are only half the story. Most states also require a separate state tax ID for:

• Sales tax collection (if you sell taxable goods or services)
• State income tax withholding (once you have employees)
• State unemployment insurance (SUTA)

Apply through your state's Department of Revenue or equivalent agency — the name and process varies. Some states share their federal EIN; others issue a distinct number. Don't assume your EIN is enough for state obligations.

The Bookkeeping Connection

Every tax ID number you hold creates a parallel paper trail. Your EIN ties to a business bank account, a set of quarterly payroll deposits, 1099s you send and receive, and an annual return. Your SSN ties to your personal 1040. Keeping these trails separate — and documented — is the difference between a smooth tax season and a scramble.

The single best habit you can build early is running all business income and expenses through accounts tied to the correct tax ID. If you get an EIN, open a bank account under that EIN and stop mixing personal and business transactions. If your clients ever issue a 1099 under the wrong ID, the IRS may flag the mismatch and send an automated notice months later. Catching this in your own ledger before the IRS catches it is far cheaper.

Good bookkeeping also preserves the option to change structures later. If you start as a sole proprietor and eventually form an LLC or S-corp, your bookkeeper (or your software) needs clean records tied to the right entity at each stage. Numbers tangled across tax IDs are one of the most expensive things to untangle retroactively.

Common Mistakes to Avoid

A few patterns that cost small business owners time, money, or both:

Paying for a free service. The IRS does not charge for EINs. Third-party sites that rank above IRS.gov in search results often charge $79 to $299 to fill out a form you can submit yourself in 15 minutes. Always go directly to IRS.gov/EIN.

Using your SSN on W-9s after you have an EIN. Once you obtain an EIN for your business, every client and vendor should receive a W-9 with the EIN — not your SSN. Update the W-9 on file with anyone who pays you.

Forgetting to update the IRS after changes. If your business name, address, ownership, or entity type changes, notify the IRS. Name changes for sole proprietors and single-member LLCs can be handled by writing the IRS; corporations and partnerships update via their annual returns. Address changes use Form 8822-B.

Applying for a second EIN for the same entity. The IRS generally issues one EIN per entity for its lifetime. Structural changes (sole prop to corporation, partnership dissolution, etc.) usually require a new EIN, but simple name or address updates do not. Applying for a duplicate can create overlapping tax accounts that are painful to unwind.

Ignoring state tax ID obligations. Getting a federal EIN does not register you with your state. If you hire employees or collect sales tax without a state ID, you can accrue penalties before you realize anything is wrong.

Letting an ITIN expire before filing. An expired ITIN doesn't stop you from owing taxes; it only stops you from claiming credits and getting refunds on time. Check renewal status before tax season starts, not the week you want to file.

Quick Decision Guide

Still unsure which number you need? Use this shortcut:

• Freelancer or sole proprietor, no employees, no LLC: SSN works. Get an EIN anyway for privacy and banking.
• Single-member LLC, no employees: SSN is legal; EIN is practical.
• Multi-member LLC, partnership, or any corporation: EIN required.
• Any business with employees, no matter how small: EIN required.
• Non-resident with U.S. tax obligations: ITIN for yourself, EIN for your U.S. business.
• Selling taxable goods or paying employees: Federal EIN plus a state tax ID.

Keep Your Records Clean from Day One

Sorting out your tax IDs is a one-time setup; using them correctly is an ongoing discipline. Every invoice, every bank deposit, every contractor payment should tie back to the right entity and the right number. Beancount.io offers plain-text accounting that keeps your books transparent, version-controlled, and AI-ready — so when tax season arrives, every transaction is already in its place. Get started for free and see why developers and finance professionals prefer plain-text accounting for the long haul.